The present invention relates to a method and apparatus for thickening and dewatering of suspensions, especially suspensions of comminuted fiber material and liquid. There are numerous prior art devices and methods for thickening suspensions, such as shown in U.S. Pat. Nos. 3,595,038 and 3,579,420, however, such prior art devices often have a number of problems associated therewith. When the particles of fibers in the suspension are small there is a tendency for the fiber material to accumulate on screening devices and the like, causing blockage thereof. While screen cleaning can be accomplished in a number of different ways, such as by the use of scrapers or doctor blades, complicated pumps and valve members for effecting backflushing, and the like, such prior art devices are generally complicated.
According to the present invention, a method and apparatus have been provided which has numerous advantages compared with most prior art structures. According to the present invention, the liquid (filtrate) separated from the suspension flows continuously away from the container through which the suspension moves, and no pressure tanks and valves, pumps, and the like are necessary for backflushing of the screens. The pressure distribution in a cylindrical screen body according to the present invention, is essentially ideal, the liquid being evenly extracted through the screen body and the pressure being evenly distributed across the screen body. Since no accessory liquid from outside sources, pumps, and the like is necessary to effect backflushing according to the present invention, pressure shocks that commonly arise in the apparatus and are transferred to the suspension are avoided. The device according to the present invention also may undergo a radial movement, which further facilitates screen cleaning, and the device according to the present invention is in general compact in design and easy to construct and maintain. The apparatus according to the present invention may readily be installed in pipe shaped containers (i.e., with diameters of about 500 millimeters) so that thickening may take place directly in pipes for transmitting the suspension. Also, according to the present invention, the suspension may be thickened despite the fact that it is at super atmospheric pressure, and at temperatures above 100.degree. C.
According to the present invention, apparatus for dewatering suspension is provided comprising a generally cylindrical container throughwhich the suspension to be dewatered flows in a first direction, screen means for removal of a portion of the liquid from the suspension flowing through the container, the screen means having an interior variable volume chamber, mechanical means for reciprocating the screen means in the first direction and in a second direction opposite to the first direction so that the volume of the interior chamber of the screen means is varied, and means connected to the interior chamber of the screen means for expelling liquid separated from the suspension to an area remote from the container. The screen means preferably comprises a tubular screen member that is slidable over a tubular member stationarily mounted to the container. A rod member, supported by bushings in the container and the stationary tubular member, extends through the stationary tubular member and is connected to the tubular screen member at a closed end portion thereof. The rod member comprises the mechanical means for reciprocating the screen means, and the rod member may be connected to a hydraulic cylinder or the like. Preferably, the screen means is moved more quickly in the second direction than in the first direction so that backflushing of the screen member takes place. Separated liquid is continuously removed from the container, even during movement of the screen member in the second direction. Also, the screen member may be rotated during movement in the second direction to further facilitate screen cleaning.
According to the method of the present invention, dewatering of the suspension of comminuted fiber material and liquid may be effected; according to the method of the present invention the following steps are effected: A flow of suspension of comminuted fiber material and liquid in a first direction in a container is established, reciprocal screen means having a variable volume interior chamber is mechanically moved in the first direction at about the speed of flow of suspension in the first direction so that liquid to be removed from the suspension flows through the screen means into the interior chamber thereof, the separated liquid is continuously removed from the container to an area remote from the container, the screen means is mechanically moved in a second direction opposite to the first direction to decrease the volume of the interior chamber of the screen means while continuously effecting removal of the separated liquid from the container to an area remote from the container, and the above method steps are continuously repeated while continuously effecting removal of separated liquid from the container. The suspension may be at super atmospheric pressure when flowing in the container and at a temperature above 100.degree. C., and preferably the suspension is thickened from a solids concentration of about 2-6% to 8-12%.
It is the primary object of the present invention to provide an improved method and apparatus for the thickening of a suspension of comminuted fiber material and liquid. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and the appended claims.